Light beam scanning apparatus and image forming apparatus

ABSTRACT

A synchronizing circuit and a timing adjusting circuit set a position of an exposure area by beam scanning as an additional value of a first setting value set in dot unit corresponding to the scanning of a pixel clock period and a second setting value set in tap unit corresponding to the scanning of a period smaller than the pixel clock period. A CPU controls the synchronizing circuit and the timing adjusting circuit so that the exposed position becomes an objective position. The CPU changes the second setting value to a first predetermined value by increasing the first setting value by one dot, in the case that the additional value is increased to reach a first threshold. Further, the CPU changes the second setting value to a second predetermined value by decreasing the first setting value by one dot, in the case that the additional value of the synchronizing circuit and the timing adjusting circuit in which the first setting value is increased by one dot is decreased to reach a second threshold smaller than the first threshold.

The present application is a continuation of U.S. application Ser. No.10/446,105, filed May 28, 2003, the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to an image forming apparatus such as adigital copying machine, in which simultaneous scanning-exposure isperformed onto a photoconductor drum with a plurality of laser beams toform an electrostatic latent image on the photoconductor drum duringcopy.

In recent years, various digital copying machines, which form an imageby scanning-exposure with a laser beam and an electrophotographicprocess, have been developed. Recently, in order to realize high-speedimage formation, a multi-beam type of digital copying machine, i.e. adigital copying machine in which a plurality of laser beams is generatedand the simultaneous scanning-exposure is performed in a plurality oflines with the plurality of laser beams has been developed.

The multi-beam type of the digital copying machine includes a pluralityof semiconductor laser oscillators which generates the laser beam, agalvanomirror which controls a position in a sub-scanning direction ofeach laser beam outputted from the plurality of semiconductor laseroscillators, a polygon mirror which scans the photoconductor drum witheach laser beam by further reflecting each laser beam reflected by thegalvanomirror toward the photoconductor drum, and a lens system unitwhich mainly includes a collimator lens and an f-θ lens.

In order to form an image at the accurate position on a paper, exposurepoints in a main scanning direction and a sub-scanning direction of eachlaser beam must be accurately adjusted in the case of the multi-beamtype of digital copying machine. The adjustment of a beam scanningposition is usually performed at the time of so-called a warming-upstate when a power supply is turned on to initialize each portion of theapparatus, at the time of so-called a standby state when the warming-upis completed and the apparatus exists at the state in which the copy canbe performed, and at the time immediately before a copy start button ispressed and the copy is started.

In the above-described multi-beam type of digital copying machine, thebeam position in the sub-scanning direction and the beam exposureposition in the main scanning direction are controlled for each laserbeam, and a positional error of each laser beam is adjusted withinseveral micron-meters. The adjustment of the beam position in thesub-scanning direction is performed by giving an indicated value to thegalvanomirror until the error becomes not larger than tolerance. Theadjustment of the beam exposure position in the main scanning directionis performed by using a pixel clock generating circuit and a delaycircuit which delays the pixel clock in unit of a fraction of one pixelexposure time. Generally since it takes relatively long processing timeto adjust the beam scanning position of the multi-beam type of digitalcopying machine, it is desired that the processing time of theadjustment is reduced.

BRIEF SUMMARY OF THE INVENTION

It is an object of the invention to reduce adjusting time of a beamexposure position in a main scanning direction and to comprehensivelyimprove processing performance of a copy of an apparatus.

In order to achieve the above object, according to one aspect of theinvention, there is provided a light beam scanning apparatus including abeam generating portion which generates a light beam, a scanning portionwhich reflects the light beam generated from the beam generating portiontoward a scanned surface and exposes the scanned surface by scanning thescanned surface in a main scanning direction with the light beam, anexposure position setting portion setting a position of an exposure areaby beam scanning of the scanning portion as an additional value of afirst setting value which is set in a first unit corresponding to thescanning of a pixel clock period and a second setting value which is setin a second unit corresponding to the scanning of the period smallerthan the pixel clock period, a first control portion which controls thebeam generating portion and the scanning portion so as to expose thearea set by the exposure position setting portion, a detecting portionwhich detects a position in the main scanning direction of the areaexposed by the first control portion, and a second control portionincluding a first changing portion which changes the second settingvalue to a first predetermined value by increasing the first settingvalue by one unit in the case that the additional value of the exposureposition setting portion is increased to reach a first threshold so thatthe exposure position detected by the detecting portion becomes anobjective position, and a second changing portion which changes thesecond setting value to a second predetermined value by decreasing thefirst setting value by one unit in the case that the additional value ofthe exposure position setting portion in which the first setting valuehas been increased by one unit is decreased to reach a second thresholdsmaller than the first threshold, wherein the second threshold isseparated from the first threshold by a distance corresponding to avalue in which the first predetermined value is added to the value ofthe second unit corresponding to the scanning of the pixel clock period.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a block diagram showing a structure of a digital copyingmachine as an image forming apparatus according to a first embodiment ofthe invention;

FIG. 2 shows a configuration of an optical system unit 13 and apositional relationship between the optical system unit 13 and aphotoconductor drum 15;

FIG. 3 shows a control system mainly containing control of a multi-beamoptical system;

FIG. 4 is an exploded view of a light receiving portion of aconventional detecting sensor 38 for beam passing position which is usedin the multi-beam optical system;

FIG. 5 mainly shows the configuration for setting an exposure area infine unit not more than one-pixel-clock period;

FIG. 6 is a view for illustrating a synchronous clock;

FIG. 7 shows the case in which an area between sensors SA and SR on alight beam detecting device 38 is irradiated with five-pixel-clockbeams;

FIG. 8 is a block diagram in which the configuration of a main scanningcontrol portion is integrally shown;

FIG. 9 is a view for illustrating a tap value corresponding to one dot;

FIG. 10 shows a flow chart for determining the tap value correspondingto one dot;

FIGS. 11A and 11B are views for illustrating main scanning control inthe case that a beam emitting timing is changed by temperature change orthe like;

FIG. 12 is a view for illustrating carry-up/carry-down of the dot andthe tap;

FIG. 13 is a flow chart showing a main scanning control routine duringstandby and control between papers;

FIG. 14 is a view for illustrating a principle of thecarry-up/carry-down of the dot and the tap according to a firstembodiment of the invention;

FIG. 15 is a view for illustrating operation of the carry-up/carry-downof the dot and the tap according to the first embodiment of theinvention;

FIG. 16 is a flow chart showing the operation according to the firstembodiment of the invention;

FIG. 17 is a view for illustrating the carry-up/carry-down of the dotand the tap according to a second embodiment of the invention;

FIG. 18 is a flow chart showing the operation according to the secondembodiment of the invention;

FIG. 19 is a view for illustrating a relationship between the dot andthe tap according to a third embodiment of the invention;

FIG. 20 is a flow chart showing the operation according to the thirdembodiment of the invention; and

FIG. 21 is a flow chart showing a main scanning control operation duringstandby and control between papers according to the third embodiment ofthe invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a block diagram showing a structure of a digital copyingmachine as an image forming apparatus according to a first embodiment ofthe invention. The digital copying machine includes a scanner portion 1as image reading means and a printer portion 2 as image forming means.

The scanner portion 1 includes a first carriage 3 and a second carriage4 which are movable in a direction of an arrow shown in FIG. 1, afocusing lens 5, and a photoelectric conversion element 6. An original Ois placed downward on an original placement plate 7 made of transparentglass, and a right side of a front in the direction of a smaller side ofthe original placement plate 7 is set to a center reference in a placingreference of the original O. The original O is pressed on the originalplacement plate 7 with an original fixing cover 8 provided openable andclosable.

The original O is irradiated with a light source 9, its reflected lightis formed to be condensed at a light receiving surface of thephotoelectric conversion element 6 through mirrors 10, 11, and 12 andthe focusing lens 5. The image of the original O placed on the originalplacement plate 7 is orderly read in each one line with the scannerportion 1, and output of the reading is converted into a digital imagesignal of 8 bits showing density of the image in an image processingportion (not shown).

The printer portion 2 includes an optical system unit 13 and an imageforming portion 14 which incorporates an electrophotographic methodcapable of forming the image on paper P which is an image formingmedium. That is to say, the image signal which is read from the originalO with the scanner portion 1 is converted into the laser beam(hereinafter referred to as light beam) from the semiconductor laseroscillator after the processing is performed in the image processingportion (not shown). In the embodiment, the multi-light beam opticalsystem using a plurality of semiconductor laser oscillators (two ormore) is adopted.

A plurality of light beams outputted from the optical system unit 13 isfocused at a point of the exposure position X on the photoconductor drum15 of an image support as scanning light of a spot having necessaryresolution and scan-exposed. Accordingly, an electrostatic latent imageis formed on the photoconductor drum 15 according to the image signal.

An electrostatic charger 16 which charges its surface, a developer 17, atransfer charger 18, a peeling-off charger 19, and a cleaner 20 areprovided in the vicinity of the photoconductor drum 15. Thephotoconductor drum 15 is rotated at predetermined circumferential speedby a driving motor (not shown) and charged by the electrostatic charger16 provided opposite to the surface of the photoconductor drum 15. Theplurality of light beams is focused in the form of the spot at the pointof the exposure position X on the charged photoconductor drum 15.

The electrostatic latent image formed on the photoconductor drum 15 isdeveloped with toner (developing agent) from the developer 17. By takingthe timing at the point of the transfer position by a paper-feedingsystem, the photoconductor drum 15 in which the toner image is formed bythe development is transferred by the transfer charger 18 onto the paperP supplied from a paper-feeding cassette 21. The toner image is fixedwith a fixing device 26 in the paper P to which the toner image istransferred, and then the paper P is discharged to an externaldischarging tray 28 through a discharging roller 27.

The optical system unit 13 will be described below.

FIG. 2 shows the configuration of the optical system unit 13 and thepositional relationship between the optical system unit 13 and thephotoconductor drum 15. For example, the optical system unit 13 containsthe semiconductor laser oscillators 31 a to 31 d, and the high-speedimage formation can be achieved without excessively increasing thenumber of rotations of the polygon mirror in such a manner that each ofthe semiconductor laser oscillators 31 a to 31 d simultaneously formsimage in one line by one line.

That is to say, a laser driver 32 a drives the semiconductor laseroscillators 31 a, the outputted light beam is incident to agalvanomirror 33 a as light path changing means after the light beampasses through the collimator lens (not shown). The light beam reflectedby the galvanomirror 33 a passes through half mirrors 34 a and 34 b andis incident to a polygon mirror 35 as a multi-surface rotating mirror.

The polygon mirror 35 is rotated at constant speed by a polygon motor 36driven with a polygon motor driver 37. Accordingly, the reflected lightfrom the polygon mirror 35 is scanned in the constant direction atangular speed determined by the number of rotations of the polygonmirror 36. The light beam scanned by the polygon mirror 35 passesthrough the f-θ lens and scans a light receiving surface of a light beamdetecting device 38 as light beam passing-detecting means and light beamposition detecting means and the photoconductor drum 15 by f-θcharacteristics of the f-θ lens. Description concerning other threelight beams is omitted due to the same configuration.

Each laser beam outputted from the individual semiconductor laseroscillators 31 a to 31 d is synthesized with half mirrors 34 a, 34 b,and 34 c and four light beams advance toward the direction of thepolygon mirror 35.

Accordingly, the four light beams can simultaneously scanned on thephotoconductor drum 15, in the case that the number of rotations of thepolygon mirror 35 is the same, the four light beams can record the imageat four-time speed as compared with a conventional single beam.

The galvanomirrors 33 a to 33 d adjust (control) the positionalrelationship between the light beams in the sub-scanning direction, andgalvanomirror diving circuits 39 a to 39 d which drives eachgalvanomirror are connected to the galvanomirrors 33 a to 33 d.

The light beam detecting device 38 detects the passing position, passingtiming, and power of the four light beams and is provided in thevicinity of an end portion of the photoconductor drum 15 so that thesurface of the light beam detecting device 30 is equivalent to thesurface of the photoconductor drum 15. The control of the galvanomirrors33 a to 33 d (control of image forming position in the sub-scanningdirection), the control of emitting power (intensity) of thesemiconductor laser oscillators 31 a to 31 d, and the control of lightemitting timing (control of image forming position in the main scanningdirection), which correspond to each light beam, are performed on thebasis of a detecting signal from the light beam detecting device 38. Inorder to generate the signal for performing the control, alight-beam-detecting-device output processing circuit 40 is connected tothe light beam detecting device 38.

Next, a control system will be described below.

FIG. 3 shows the control system mainly containing the control of themulti-beam optical system. That is to say, a numeral 51 is a maincontrol portion which performs the general control, for example the maincontrol portion includes a CPU and comprehensively controls a memory 52,a control panel 53, an external communication interface (I/F) 54, thelaser drivers 32 a to 32 d, the polygon mirror motor driver 37, thegalvanomirror driving circuits 39 a to 39 d, thelight-beam-detecting-device output processing circuit 40 as signalprocessing means, a synchronizing circuit 55, an image data interface(I/F) 56, and the like.

The image data I/F 56 is connected to the synchronizing circuit 55, andan image processing portion 57 and a page memory 58 are connected to theimage data I/F 56. The scanner portion 1 is connected to the imageprocessing portion 57 and an external interface (I/F) 59 is connected tothe page memory 58.

At this point, when a flow of the image data in forming the image issimply described, the flow is as follows;

In the case of copying operation, as described above, the image of theoriginal O set on the original placement plate 7 is read with thescanner portion 1 and sent to the image processing portion 57. The imageprocessing portion performs, e.g. shading correction, various kinds offiltering, gray level processing, and gamma correction, which are known,to the image signal from the scanner portion 1.

The image data from the image processing portion 57 is sent to the imagedata I/F 56. The image data I/F 56 divide the image data among the fourlaser drivers 32 a to 32 d.

The synchronizing circuit 55 generates a clock synchronized with thetiming which each light beam passes through the light beam detectingdevice 38. In synchronization with this clock, the synchronizing circuit55 sends image data as a laser modulating signal from the image data I/F56 to each of the laser drivers 32 a to 32 d. The control panel 53 is aman-machine interface which performs starting of the copying operation,setting of the number of copying paper, or the like.

In this manner, the image formation which is synchronized in the mainscanning direction, i.e. the image formation into the correct positionis performed by transferring the image data while the transfer issynchronized with the scanning of each light beam.

The galvanomirror driving circuits 39 a to 39 d are the circuits whichdrive the galvanomirrors 33 a to 33 d according to the indicated valuefrom the main control portion 51. Accordingly, the main control portion51 can freely control each angle of the galvanomirrors 33 a to 33 dthrough the galvanomirror driving circuits 39 a to 39 d. The passingposition in the sub-scanning direction of the light beam is detectedwith the light beam detecting device 38 which will be described later,and the main control portion 51 controls the galvanomirror drivingcircuits 39 a to 39 d so that the passing position becomes the desiredposition.

The polygon motor driver 37 is the driver which drives the polygon motor36 for rotating the polygon mirror 35 scanning the four light beamsdescribed above. The main control portion 51 can perform start and stopof the rotation and switching of the number of rotations for the polygonmotor driver 37. The switching of the number of rotations is performedin changing a recording pitch (resolution).

The laser drivers 32 a to 32 d generate the laser beam according to thelaser modulating signal synchronized with the scanning of the light beamfrom the synchronizing circuit 55 described above and have a function offorcedly and individually emitting the semiconductor laser oscillators31 a to 31 d according to a forced light-emitting signal from the maincontrol portion 51 while the forced light-emitting operation isirrelevant to the image signal.

This function is adopted in forcedly emitting operation of each of thelaser oscillators 31 a to 31 d in the case of the scanning-exposurepositional control by the light beam or execution of light beam powercontrol, which are described later.

The main control portion 51 sets light-emitting power of thesemiconductor laser oscillators 31 a to 31 d for the laser drivers 32 ato 32 d. The setting of the light-emitting power is changed according toa change in process conditions or the passing position detection of thelight beam. Each of the light-emitting power of the semiconductor laseroscillators 31 a to 31 d is controlled so as to be equal. The control ofthe light-emitting power is preformed prior to the control of the mainscanning direction and the control of the sub-scanning direction of thelight beam, which are shown below. Unless the control of thelight-emitting power is performed at first in the light beam control,the light-emitting power of each light beam is different and the controlof the main scanning direction and the control in the sub-scanningdirection of the light beam can not be performed.

FIG. 4 is the exploded view of the light receiving portion of the sensor38 for detecting beam passing position which is used in the multi-beamoptical system. Sensors SA, SB, S, SD, SI, SJ, SK, SL, SM, SN, SH, SQ,and SR are made by the photoelectric conversion element (photodiode),and the sensors output current proportional to light amount when thesensors receive the light. Main scanning control function in the lightreceiving portion related to the invention will be mainly describedbelow.

The beam passing timing sensor SA is used with the beam passing timingsensor SB and generates a reference signal of the main scanning controland the beam passing timing control. The beam passing timing sensor SRis one for adjusting the beam passing timing in the case of the mainscanning control. In the main scanning control, the light-emittingtiming is controlled so that the plurality of scanning beams passthrough the position of the timing sensor SR after the scanning beamspass through the timing sensor SB.

Then, beam light-emitting timing control will be described. For example,the beam light-emitting timing control is to control the light-emittingtiming of the laser beam so that the exposure positions of themulti-beam correspond in the sub-scanning direction, in the case that astraight line is formed in the sub-scanning direction by using themulti-beam.

FIG. 5 mainly shows the configuration for setting a printing area(exposure area) in fine unit not more than one-pixel-clock period(corresponding to one dot on the paper), corresponds to thesynchronizing circuit 55, the light-beam-detecting-device outputprocessing circuit 40, and the light beam detecting device 38 in theblock diagram shown in FIG. 3.

In FIG. 5, a main-scanning direction light-beam-detecting-device outputprocessing circuit 40 a includes a first counter 111 and a secondcounter 112, and a latch circuit 113.

The synchronizing circuit 55 includes four quartz oscillators 114 a to114 d, a selector 115 which selects the quartz oscillators 114 a to 114d, a clock synchronizing circuit 116, a delay line 117, four delay clockselectors 118 a to 118 d, four image-transfer-clock generating portions(printing area setting portion) 119 a to 119 d, a sample timer 120, anOR gate circuit 121, and a on-drum light-emission prohibiting timer 122.One of the four quartz oscillators 114 a to 114 d is selected with theselector 115 corresponding to the image resolution.

As shown in FIG. 5, the sensor SA of the light beam detecting device 38is exposed by a front-end beam among light beams a, b, c, and d whichare forcedly emitted by the sample timer 120, and a signal output levelbecomes “H” (high level) from “L” (low level). The signal is inputted tothe sample timer 120 and the forced light-emission of the semiconductorlaser oscillators 31 a to 31 d is entirely released.

Accordingly, the light beams a, b, c, and d are eliminated and the highoutput of the sensor SA is eliminated.

The output of the sensor SA is also inputted to the clock synchronizingcircuit 116 in the synchronizing circuit 55. The clock synchronizingcircuit 116 outputs the synchronous clock. As shown in FIG. 6, thesynchronous clock is the clock which rises from a trailing edge of theoutput of the sensor SA with delay of ΔT_(SYNC). The synchronous clockis also the clock which synchronizes with the output of the sensor SAand has the same frequency as the output clock of the quartz oscillator.

Then, the synchronous clock is inputted to the delay line 117. The delayline 117 has the function of delaying the inputted signal forpredetermined time. The delay line 117 shown in FIG. 5 has ten taps asthe output. That is to say, a delay clock D1 outputted from a first steptap becomes the clock delayed by Δtd relative to the inputtedsynchronous clock, and a delay clock D2 outputted from a second step tapbecomes the clock delayed by 2·Δtd relative to the inputted synchronousclock. The delay Δtd is actually several nano-seconds.

A delay clock D10 outputted from a final step (tenth step) tap becomesthe clock delayed by 10·Δtd relative to the inputted synchronous clock.In the embodiment, one-tenth of one period of the synchronous clock isalmost equal to Δtd. That is to say, the delay clock D10 has almost thesame phase for the inputted synchronous clock and is shifted by oneclock relative to the inputted synchronous clock.

In the embodiment, though the delay amount of the delay line 117 is setto one-tenth of one clock, in the case that the setting of the printingarea is required more accurately, the delay amount per one tap may bedecreased and the number of taps may be increased.

The output from the delay line 117, i.e. the delay clocks D1 to D10 areinputted to the delay clock selectors 118 a to 118 d corresponding tothe light beams a to d. The function of the delay clock selectors 118 ato 118 d selects the clock, which is outputted to theimage-transfer-clock generating portions (printing area setting portion)119 a to 119 d of the next step, on the basis of a delay clock selectingsignal outputted from the main control portion 51 for each selector. Inother words, the main control portion 51 can freely select the clock forsetting the printing area among the delay clocks D1 to D10 in each ofthe light beams a to d.

Next, the image-transfer-clock generating portions (printing areasetting portion) 119 a to 119 d will be described below. The maincontrol portion 51 sets the printing area in each of the light beams ato d in one clock unit (one dot or one pixel unit) by using a printingarea setting signal. That is to say, the main control portion 51 can setthe output timing and the number of outputs of the image transfer clockfor each light beam. In the case of the usual image formation, asdescribed above, the image forming area of each of the light beams a tod is set so as to become the objective image forming area on thephotoconductor drum 15. The objective image forming area is changed bythe size of the paper used or the setting of a binding space.

The image transfer clock (printing area signal) obtained in theabove-describe way is sent to the image data I/F 56, and the image data(laser modulating signal) corresponding to each of the light beams a tod is outputted synchronizing with the image transfer clock (printingarea signal), i.e. at an image transfer clock period. Accordingly, animage transfer clock signal CK is also referred to as a pixel clock.

The laser drivers 32 a to 32 d modulate the semiconductor laseroscillators 31 a to 31 d with the image data (laser modulating signal).The image transfer clock signal CK outputted from theimage-transfer-clock generating portions 119 a to 119 d in adjusting theexposure position in the main scanning direction is used as the pixelclock for the exposure. The corresponding semiconductor laser oscillator31 is forcedly emitted during a period of the clock generation.

The main control portion 51 can set the printing area in one clock unit(one dot unit) with the printing area setting signal for theimage-transfer-clock generating portions 119 a to 119 d. Further, themain control portion 51 can set the printing area in one-tenth clockunit (one-tenth dot unit) with the delay clock selecting signal for eachof the delay clock selectors 118 a to 118 d while the printing area isindependent of each of the light beams a to d. The detailed descriptionof this circuit operation is disclosed in U.S. Pat. No. 6,208,367 “LightBeam Scanning Apparatus and Image Forming Apparatus” assigned to thesame assignor of the invention.

The schematic operation of beam light-emitting timing control will besimply described.

FIG. 7 shows the exposure area EA5 in which the area between the sensorsSA and SR on a light beam detecting device 38 is irradiated with thefive-pixel-clock beams. A sign of r is the minimum amount of adjustmentin the main scanning direction and corresponds to the delay time of onetap of the delay line 117, i.e. the minimum delay time. As shown in FIG.7, the delay clock is generated after a sensor pattern SA is started tobe irradiated with the light beam and a predetermined time elapses. Aperiod Pc is equal to the period of the image transfer clock, i.e. thepixel clock CK. A distance R is the distance of one dot. Assuming thatthe scanning speed on the scanned surface of the light beam is set to Vs(m/sec) and the clock period is set to Pc (sec), then R=Vs·Pc (m).Assuming that the delay time of one tap is set to Δtd, the minimumamount of adjustment r becomes r=Vs·Δtd (m).

For example, as shown in FIG. 7, delay time Td from a rising point T1 ofthe pixel clock immediately before the five-pixel exposure area EA5 isadjusted so that a front-end portion of the exposure area EA5 is incontact with the sensor SR or partially overlapped with the sensor SR.The exposure position on the photoconductor drum is determined on thebasis of the number of clocks and the delay time Td after thepredetermined delay time, which are determined for each light beam. Theexposure positions in the main scanning direction of the light beams ato d coincide with each other by performing the above-described controlto the light beams a to d. In the copying machine of the embodiment, thebeam position in the main scanning direction can be adjusted in unit ofa fraction of one pixel.

In the main scanning control, the main control portion 51 first turns onall the light beams and scans them, turns off all the light beams afterthe front-end light beam passes through the timing sensor SA, emits onlythe light beam to be an object immediately before the timing sensor SAfor, e.g. five dots, and monitors the output of the timing sensor SR.The setting of the dot value “5” is performed at the image interface 56,and the setting of the delay amount is performed at the delay line 117,the delay clock selector 118 (delay in tap unit), and theimage-transfer-clock generating portion 119 (delay in dot unit). Whenthe output from the timing sensor SR exists, the light beam is emittedin the vicinity of the timing sensor SR.

For example, an eight-face polygon mirror is used and the output of thetiming sensor SR of two rotations of the polygon mirror is counted withthe counter 112. Management of the two rotations of the polygon mirrorcan be performed by counting the signal generated from the output of thesensors SA and SR. In the case that the output of the timing sensor SRis outputted in 16 times (eight-face×two rotations), the light beam isemitted on the timing sensor SR. As the number of outputs of the sensorSR is decreased from 16, the beam light-emitting timing is separatedfrom the sensor SR. In the case that the number of outputs of the timingsensor SR is zero, the light beam is not emitted on the timing sensorSR. The positional relationship between the beam light-emitting timingand the sensor SR can be recognized by counting the number of outputs ofthe timing sensor SR.

In the main scanning control, the exposure position (light-emittingtiming) of the light beam is adjusted in dot unit (R unit correspondingto clock period Pc) and tap unit not larger than one dot (r unit of theminimum amount of adjustment corresponding to delay amount of one tap),and the values of the dot and the tap in the case that the output of thetiming sensor SR is changed from 16 times to 15 times is set to thesetting value in the main scanning direction control.

The above-described operation is performed to all the laser beams to bethe object and the setting value of the beam light-emitting timingcontrol of each light beam is measured.

FIG. 8 is the block diagram integrally showing the configuration of themain scanning control portion. As described above, the main controlportion (hereinafter referred to as CPU) 51 adjusts the light emittingtiming of the laser beam in dot unit or in unit less than one dot (tapunit) by setting the data in the synchronizing circuit and the beamlight-emitting timing adjusting circuit 55. For example, in the casethat the minimum amount of adjustment of the tap is set to the one-tenthdot, the light-emitting timing of the laser beam can be adjusted inone-tenth dot unit. The beam light-emitting timing adjusting circuit 55shifts the exposure position in unit not larger than one dot by finelydelaying the light-emitting clock of the one dot unit.

The output of the timing sensors SA and SR is converted into a voltagevalue from a current value by I/V converters 60A and 60B, binarized bybinarizing circuits 61A and 61B, and then outputted to a timing signalgenerating circuit 62. The timing signal generating circuit 62 generatesthe timing signal from the outputs of the sensors SA and SB, and thetiming signal is inputted to the CPU 51, the synchronizing circuit, andthe timing adjusting circuit 55.

The output of the timing sensor SR is converted into the voltage valueby an I/V converter 60R, binarized by a binarizing circuit 61R, and thenoutputted to the counter 112. The counter counts the number of outputsof the timing sensor SR within the predetermined time for the CPU 51.For example, the CPU 51 can recognize how many times of output signalsof the timing sensor SR are generated within the two rotations of theeight-face polygon mirror 35. In the case that the light beam is emittedon the timing sensor SR, a count value of 8×2=16 is outputted.

As described above, the shift in the main scanning direction by the mainscanning control is corrected in unit not greater than one dot. However,since the delay time of each tap due to a change in temperature and thelike is not constant, it is necessary that the shift of the mainscanning timing is monitored and the tap value is always corrected.Further, since the number of taps is finite, the carry-up operation isrequired of dot value and tap value. That is to say, in the case thatthe tap value corresponding to one dot is previously determined, the tapvalue reaches the tap value corresponding to one dot (one pixel clockperiod), and the tap value is increased by one tap, one is added to thedot value and the tap value is set to zero (carry-up). In the case thatthe tap value is zero and the tap value is decreased, the dot value isdecreased by one and the tap value is set to “the tap value of −1corresponding to one dot” (carry-down). The shift of the main scanningexposure position is always corrected by the above-described operation.

FIG. 9 is the view for illustrating the operation for determining thetap value corresponding to one dot.

The exposure position indicated by EPm in FIG. 9 shows the position inwhich the output from the timing sensor SR is outputted by thepredetermined number at m dot after passing through the timing sensorSB. For example, the position in which the count value of 16 of thecounter 112 is outputted to the CPU 51 is shown. That is to say, the CPU51 increases the dot value until the exposure position becomes thisposition. Circles shown in FIG. 9 indicate a rearmost exposed area whenthe five-dot area is exposed with the light beam.

One dot is subtracted from m dot, the counter value of the timingcounter SR is read, and the tap value is increased until it becomes thepredetermined value. When the count value reaches the predeterminedvalue, the dot value and the tap value at that time are stored in thememory. For example, assuming that the tap value is i tap at this point,(m−1 dot, i tap) is stored in the memory.

Then, one dot is subtracted from (m−1) dot, the counter value of thetiming sensor SR is read, and the tap value is increased until itbecomes the predetermined value. For example, assuming that the tapvalue is j tap at this point, (m−2 dot, j tap) is stored in the memory.

When the above-described difference is calculated, the tap valuecorresponding to one dot is determined.

-   -   one dot=(j−i) tap

FIG. 10 is the flow chart of the operation.

At first, the CPU 51 sets the dot value and the tap value of each lightbeam to zero (STEP 1). In the embodiment, though the dot value is set tozero, a proper value may be set from the positional relationship betweenthe timing sensor SB and the timing sensor SR. In that case, there is anadvantage that executing time of the measuring routine is reduced.

The CPU 51 emits all the light beams, and turns on the laser beam to bemeasured and turns off the laser beam not to be the measuring objectwhen the output of the timing sensor SB is outputted (STEP 4). Then, theCPU 51 decides whether the count value of output of the timing sensor SRis the predetermined value or not (STEP 5). If the count value is notthe predetermined value, the flow is moved to STEP 6. If the count valueis the predetermined value, the flow is moved to STEP 7.

In STEP 6, the CPU 51 increases the dot to be emitted by one dot. Whenthe dot is increased by one dot, the light-emitting timing is shifted tothe downstream side in the light beam scanning direction by one dot.That is to say, the light-emitting timing approaches the timing sensorSR. The CPU 51 repeats the operation of STEP 5 and STEP 6 until thecount value of the timing sensor SR becomes the predetermined value, andthe CPU 51 adjusts the light-emitting timing (increases the dot value)until the exposure position reaches the position on the timing sensorSR. In STEP 7, the CPU 51 stores the dot value at that time in thememory.

In STEP 8, the CPU 51 decreases the dot to be emitted by one dot (seeposition of “m−1” dot in FIG. 8). In STEP 9, similarly to STEP 5, theCPU 51 decides whether the count value of output of the timing sensor SRis the predetermined value or not. If the count value is not thepredetermined value, the flow is moved to STEP 10. If the count value isthe predetermined value, the flow is moved to STEP 11.

In STEP 10, the CPU 51 leaves the dot to be emitted and increases thetap by one tap. When the tap to be emitted is increased by one tap, theexposure position approaches the downstream side in the light beamscanning direction by one tap. That is to say, the exposure positionapproaches the timing sensor SR. The CPU 51 repeats the operation ofSTEP 9 and STEP 10 until the count value of the timing sensor SR becomesthe predetermined value, and the CPU 51 adjusts the light-emittingtiming (increases the tap value) until the light-emitting point reachesthe position on the timing sensor SR. In STEP 11, the CPU 51 stores thedot value and the tap value at that time in the memory.

In STEP 12, the CPU 51 further decreases the dot to be emitted by onedot and the tap value is set to zero (see exposure position EMm-2 dot inFIG. 9). In STEP 13, similarly to STEP 5 and STEP 9, the CPU 51 decideswhether the count value of output of the timing sensor SR is thepredetermined value or not. If the count value is not the predeterminedvalue, the flow is moved to STEP 14. If the count value is thepredetermined value, the flow is moved to STEP 15.

In STEP 14, the CPU 51 leaves the dot to be emitted and increases thetap by one tap. When the tap to be emitted is increased by one tap, thelight-emitting timing approaches the downstream side in the light beamscanning direction by one tap. That is to say, the light-emitting timingapproaches the timing sensor SR. The CPU 51 repeats the operation ofSTEP 13 and STEP 14 until the count value of the timing sensor SRbecomes the predetermined value, and the CPU 51 adjusts thelight-emitting timing (increases the tap value) until the light-emittingpoint reaches the position on the timing sensor SR. When the count valuebecomes the predetermined value, the CPU 51 stores the dot value and thetap value at that time in the memory (STEP 15). In STEP 16, the CPU 51decreases the tap value stored in the memory in STEP 11 from the tapvalue stored in the memory in STEP 15 and determines the tap valuecorresponding to one dot.

The main scanning control in the case that the beam light-emittingtiming (exposure position) is changed by the change in temperature andthe like will be described referring to FIGS. 11A and 11B. This isexecuted, e.g. during a standby state or a printing interval betweenpapers. “The printing interval between papers” shows the time from thecompletion of the printing of one paper to the start of the printing ofthe next paper during the copying. FIG. 11A shows the state in which thepredetermined value is outputted as the count value of the timing sensorSR of n dots and k taps as a result of the main scanning control. Anexposure position Eat is the objective exposure position. On the otherhand, FIG. 11B shows the state in which the tap value is increased to mtap so that the exposure position returns to the objective positionbecause the beam light-emitting timing (exposure position) is changed byinfluence of the temperature or the like and shifted to the upstreamside in the light beam scanning direction. Further, since the tap valueis m corresponding to one dot in the embodiment, the dot value isincreased by one and the tap value is set to zero (carry-up). That is tosay, the dot value is set to n+1 dot and the tap value is set to zero(see exposure position EP(n+1)).

The carry-up and the carry down of the dot and the tap will bespecifically described referring to FIG. 12. FIG. 12 is the view showingthe relationship between the dot and the tap. A vertical axis shows thedot value and a horizontal axis shows the tap value. In this case, thetap value corresponding to one dot is 10 and it is assumed that ninetaps are provided in the apparatus. When the tap value reaches 10, thedot is increased by one and the tap value is set to zero. That is tosay, when the dot value reaches n dots and the tap value reaches 10taps, the dot value is set to (n+1) dots and the tap value is set tozero (carry-up), and then the tap value is increased for (n+1) dots. Onthe contrary, in the case that the tap value is zero and furtherdecreased by one tap, the dot value is set to n dots and the tap valueis set to (10−1) taps.

FIG. 13 is the view in which the above-described operation is shown inthe flow chart. As described above, the operation is the main scanningcontrol routine of the timing other than the case of the imageformation, i.e. during the standby state and the printing intervalbetween papers.

Since the main scanning control routine is performed after the beamlight-emitting timing control has been executed once, the dot value andthe tap value which are the previous control vale are set at first (STEP20).

All the light beam are turned on (STEP 21), the light beam is scanned onthe sensor by the rotation of the polygon mirror, only the laser beam tobe measured is emitted after passing through the timing sensor SB, andthe laser beam not to be measuring object is turned off (STEP 22).

The CPU 51 reads the count value of the timing sensor SR from thecounter (STE). The CPU 51 decides whether the count value of output ofthe timing sensor SR is a predetermined value M or not (STEP 24). Forexample, in the case that the polygon mirror consists of a 12-facemirror, the predetermined value M is 6. If the count value is smallerthan the predetermined value M, the flow is moved to STEP 28. If thecount value is not smaller than the predetermined value M, the flow ismoved to STEP 25.

In STEP 25, the CPU 51 decides whether the tap value is zero or not. Ifthe tap value is zero, the flow is moved to STEP 27. If the tap value isnot zero, the flow is moved to STEP 26. For the case in which the flowis moved to STEP 25, the laser beam to be measured is placed on thetiming sensor SR. Accordingly, the CPU 51 performs the operation forshifting the light-emitting timing by one tap to the upstream side inthe scanning direction by one tap in STEP 26 or STEP 27. In that case,the CPU 51 decides whether the carry-down of the dot value and the tapvalue is required or not in STEP 25.

If the tap value is not zero, the tap value is decreased by one tap(STEP 26). If the tap value is already zero, i.e. if the tap valuereaches the minimum value, the carry-down is performed. That is to say,the CPU 51 sets the tap value to “the value corresponding to one dot −1”(9 at this point) and decreases the dot value by one (STEP 27).

In STEP 28, the CPU 51 decides whether the tap value “the valuecorresponding to one dot −1” (9 at this point) or not. If the tap valueis “the tap value corresponding to one dot −1,” the flow is moved toSTEP 29. If the tap value is “the tap value corresponding to one dot−1,” the flow is moved to STEP 30. In STEP 28, since the count value ofthe timing sensor SR is smaller than the predetermined value M, thelaser beam to be measured is not placed on the timing sensor SR or is inthe vicinity of the timing sensor SR. Accordingly, the CPU 51 performsthe operation for shifting the light-emitting timing by one tap to thedownstream side in the scanning direction in STEP 30 or STEP 28. In thatcase, the CPU 51 decides whether the carry-up of the dot value and thetap value is required or not in STEP 28.

If the tap value does not reach “the value corresponding to one dot −1,”the CPU 51 increases the tap value by one tap and shifts thelight-emitting timing by one tap to the downstream in the main scanningdirection (STEP 30). If the tap value reaches “the tap valuecorresponding to one dot −1,” the carry-up is performed. That is to say,the CPU 51 sets the tap value to zero and increases the dot value by one(STEP 29).

The above-described operation is executed for the laser beam to bemeasured and the main-scanning light-beam passing timing is alwaysfinely adjusted. However, in this method, since a threshold of thecarry-up/carry-down of the tap is one, there is a possibility that thecarry-up/carry-down occurs in vibratory. Though adjustment of the dotand the tap is performed with the electric circuit, the delay time ineach one tap is very fine and fluctuation is easily generated by thechange in environment such as the change in the temperature. In the casethat the number of dots is changed in a stroke by “the tap valuecorresponding to one dot −1” by the carry-up/carry-down operation, thereis the possibility that the delay time is changed greatly exceeding onedot. As a result, for example there is the possibility that the image,in which the shift in the main scanning direction is generated, isformed in each printing of the continuous copying.

Therefore, the embodiment provides the light beam scanning apparatus, inwhich the carry-up/carry-down of the dot and the tap is prevented fromfrequently generating during the main scanning control and the shift inthe main scanning direction is small.

FIG. 14 shows the principle of the carry-up/carry-down of the dot andthe tap according to the embodiment. The vertical axis shows the dotvalue and the horizontal axis shows the tap value. In the embodiment,three-tap hysteresis is set to the carry-up and carry-down operation asthe example. Even in this case, the tap value corresponding to one dotis 10.

When the tap value reaches 10, the dot is increased by one and the tapvalue is set to zero. That is to say, when the dot value reaches n dotsand the tap value reaches 10 taps, the dot value is set to (n+1) dotsand the tap value is set to zero (carry-up), and then the tap value isincreased for (n+1) dots. On the other hand, even if the tap value iszero in (n+1) dots, the carry-down is not performed. When the tap valuebecomes −3, the carry-down is initially performed. That is to say, thedifference of three taps is provided between the thresholds of thecarry-up and carry-down. Consequently, since the hysteresis is generatedin the carry-up/carry-down, that the carry-up/carry-down operationbecomes vibrational can be prevented.

However, there is the problem in practice for the carry-up/carry-downoperation as it is. That is to say, since the tap is not actually setnot smaller than zero, the hysteresis can not be provided. Therefore,the carry-up/carry-down operation is practically executed by adoptingthe method shown in FIG. 15.

FIG. 15 is the view showing the relationship between the dot and the tapaccording to the first embodiment of the invention. The vertical axisshows the dot value and the horizontal axis shows the tap value. Even inthis case, though the tap value corresponding to one dot is 10, the tapis provided not larger than 12 taps. In this case, when the tap valuereaches 13, the dot is increased by one and the tap value is set tothree. That is to say, when the dot reaches n dots and the tap reaches13 taps, the dot is set to (n+1) dots and the tap is set to 3 taps(carry-up), and then the tap value is increased for (n+1) dots. On theother hand, when the tap value becomes zero at (n+1) dots, thecarry-down is performed. Consequently, the difference of three taps canbe provided between the thresholds of the carry-up and carry-down.

FIG. 16 is the flow chart showing the operation according to theembodiment.

Since STEP 40 to STEP 44 are the same as STEP 20 to STEP 24 in FIG. 13,the description is omitted.

If the count value of the output of the timing sensor SR is not largerthan the predetermined value M (in the case of YES in STEP 44), the CPU51 decides whether the tap value is one or not (STEP 45). If the tapvalue is one, the flow is moved to STEP 47. If the tap value is not one,the flow is moved to STEP 46. In the case that STEP 45 is executed., thelaser beam to be measured is placed on the timing sensor SR (the outputof the timing sensor SR is outputted not smaller than the predeterminednumber of outputs). Accordingly, the CPU 51 performs the operation forshifting the light-emitting timing (exposure position) by one tap to theupstream side in the scanning direction in STEP 46 or STEP 47. In thatcase, the CPU 51 decides whether the carry-down of the dot value and thetap value is required or not in STEP 45.

If the tap value is not one, the tap value is decreased by one tap (STEP46). That is to say, the CPU 51 shifts the light-emitting timing by onetap to the upstream side in the scanning direction. If the tap value isone, i.e. if the tap value already reaches the minimum value, thecarry-down is performed (STEP 47). That is to say, the CPU 51 sets thetap value to “the value corresponding to one dot” (10 at this point) anddecreases the dot value by one. The CPU 51 may decide whether the tapvalue is zero or not in STEP 45, if the tap value is zero, the CPU 51may set the tap value to “the value corresponding to one dot −1” anddecrease the dot value by one. In that case, the hysteresis becomes fourtaps.

If the count value of the output of the timing sensor SR is smaller thanthe predetermined value M (in the case of No in STEP 44), the CPU 51decides whether the tap value is “the tap value corresponding to onedot+the amount of the hysteresis −1” (12 at this point) or not (STEP48). If the tap value is “the tap value corresponding to one dot+theamount of the hysteresis −1,” the flow is moved to STEP 50. If not, theflow is moved to STEP 49. In the case that STEP 48 is executed, sincethe count value of the timing sensor SR is smaller than thepredetermined value M, the laser beam to be measured is not placed onthe timing sensor SR or is in the vicinity of the timing sensor SR. Inthis case, the CPU 51 performs the operation for shifting thelight-emitting timing (exposure position) by one tap to the downstreamside in the scanning direction in STEP 49 or STEP 59. In that case, TheCPU 51 decides whether the carry-up of the dot value and the tap valueis required or not in STEP 48.

If the tap value does not reach “the tap value corresponding to onedot+the amount of the hysteresis −1,” The CPU 51 increases the tap valueby one and shifts the light-emitting timing by one tap to the downstreamside in the scanning direction (STEP 49). If the tap value reaches “thetap value corresponding to one dot+the amount of the hysteresis −1,” TheCPU 51 increases the tap value by three (=hysteresis value) and the dotvalue by one and performs the carry-up (STEP 50).

As described above, the image forming apparatus according to theembodiment includes an exposure position setting portion (117, 119, and56) setting the position of the exposure area exposed by the beamscanning of the scanning portion 35 as an additional value of a firstsetting value (dot value) which is set in a first unit (dot unit)corresponding to the scanning of the pixel clock period Pc and a secondsetting value (tap value) which is set in a second unit (tap unit)corresponding to the scanning of the period Δ·td smaller than the pixelclock period Pc.

The CPU 51 changes the setting of the exposure position setting portionaccording to the shift from the objective position of the exposureposition EP during the standby state or the printing interval betweenpapers and corrects the shift.

The CPU 51 includes a first changing portion (STEP 50) which increasesthe first setting value (dot value) by one unit and changes the secondsetting value (tap value) to a first predetermined value (3) in the casethat the additional value of the exposure position setting portion isincreased to reach a first threshold (n dots and 13 taps), and a secondchanging portion (STEP 27) which decrease the first setting value (dotvalue) by one unit and changes the second setting value (tap value) to asecond predetermined value (10) in the case that the additional value ofthe exposure position setting portion, in which the first setting valueis increased by one unit, is decreased to reach a second threshold (n+1dots and 0 tap) smaller than the first threshold.

A second embodiment of the invention will be described below.

As described in the first embodiment, there is the possibility that themain scanning shift occurs in the carry-up/carry-down of the dot and thetap. In the second embodiment, frequency of the main scanning shift isreduced. In the second embodiment, the carry-up is not performed untilthe tap value in control becomes a predetermined integer multiple valueof the tap value corresponding to one dot, and only the tap value isincreased.

FIG. 17 is the view showing the relationship between the dot and the tapaccording to the second embodiment. The vertical axis shows the dotvalue and the horizontal axis shows the tap value. At this point, thoughthe tap value corresponding to one dot is 10, the case in which the tapsfor P dots (for example, three dots) are provided will be described.That is to say, the 10P (=30) taps are provided.

In the case that the position of the exposure area of the light beam isshifted by the change in the temperature and it is necessary to increasethe tap value, the tap value is increased by one tap by one tap from thepresent tap value. In the conventional method, in the case that the tapvalue reaches the tap value corresponding to one dot, the carry-up ofthe dot is performed. However, in the embodiment, the carry-up is notperformed until the tap value becomes 30 and only the tap value isincreased. In FIG. 17, the objective position of the control, i.e. acentral value is (n−1) dots and +16 taps. This position corresponds tothe conventional n dots and 6 taps.

Since the delay time of one tap is very small, when the tap is increasedone by one, the frequency of the large error in the main scanningdirection is lessened unlike the conventional carry-up. However, sincethe number of taps is not infinite, the carry-up is performed when thetap value becomes the predetermined value. The predetermined value isdetermined by the relationship between the tap value of one dot and theamount of shift of the exposure position.

FIG. 18 is the flow chart showing the operation of the embodiment.

Since STEP 50 to STEP 54 are the same as STEP 20 to STEP 24 in FIG. 13,the description is omitted.

If the count value of the output of the timing sensor SR is not smallerthan the predetermined value M (in the case of YES in STEP 64), the CPU51 decides whether the tap value is zero or not (STEP 65). If the tapvalue is zero, the flow is moved to STEP 67. If the tap value is notzero, the flow is moved to STEP 66. In the case the STEP 65 is executed,the laser beam to be measured is placed on the timing sensor SR.Accordingly, the CPU 51 performs the operation for shifting thelight-emitting timing (exposure position) by one tap to the upstreamside in the scanning direction in STEP 66 or STEP 67. In that case, theCPU 51 decides whether the carry-down of the dot value and the tap valueis required or not in STEP 65.

If the tap value is not zero, the CPU 51 decreases the tap value by onetap (STEP 66). If the tap value is zero, i.e. if the tap value alreadyreaches the minimum value, the carry-down is performed (STEP 67). Thatis to say, the CPU 51 sets the tap value to “the value (10P) obtained bymultiplying the value corresponding to one dot (10 at this point) by theinteger (3 at this point) −1” and decreases the dot value by P.

If the count value of the output of the timing sensor SR is smaller thanthe predetermined value M (in the case of No in STEP 64), the CPU 51decides whether the tap value is “the value (=10P) obtained bymultiplying the value corresponding to one dot (10 at this point) by theinteger P (3 at this point)” or not (STEP 68). If the tap value is 10P,the flow is moved to STEP 70. If not, the flow is moved to STEP 69. Inthe case that STEP 68 is executed, since the count value of the timingsensor SR is smaller than the predetermined value M, the laser beam tobe measured is not placed on the timing sensor SR or is in the vicinityof the timing sensor SR. In this case, the CPU 51 performs the operationfor shifting the light-emitting timing by one tap to the downstream sidein the scanning direction in STEP 69 or STEP 70. In that case, the CPU51 decides whether the carry-up of the dot value and the tap value isrequired or not in STEP 68.

If the tap value does not reach 10P, the CPU 51 increases the tap valueby one and shifts the light-emitting timing by one tap to the downstreamside in the scanning direction (STEP 69). If the tap value reaches 10P,the CPU 51 sets the tap value to one, increases the dot value by P(three at this point), and performs the carry-up (STEP 70).

As described above, the image forming apparatus according to the secondembodiment includes the exposure position setting portion (117, 119, and56) setting the position of the exposure area EA exposed by the beamscanning of the scanning portion 35 as the additional value of the firstsetting value (dot value) which is set in the first unit (dot unit)corresponding to the scanning of the pixel clock period Pc and thesecond setting value (tap value) which is set in the second unit (tapunit) corresponding to the scanning of the period Δ·td smaller than thepixel clock period Pc.

The CPU 51 changes the additional value of the exposure position settingportion according to the shift from the objective position of theexposure area EA during the standby state or the printing intervalbetween papers and corrects the shift.

The CPU 51 includes a changing portion (STEP 70) which increases thefirst setting value (dot value) by the predetermined number (P) andchanges the second setting value (tap value) in the case that the secondsetting value (tap value) is increased to the value more than the value(30) corresponding to a distance of a predetermined multiple (P) of thefirst unit (dot unit).

A third embodiment of the invention will be described below.

In the second embodiment, though the frequency of thecarry-up/carry-down is decreased, when the carry-down or the like isgenerated, there is the possibility that the relatively large error isgenerated in the main scanning direction. In the third embodiment, theerror of the light beam positioning in the main scanning direction isdecreased as small as possible in such a manner that thecarry-up/carry-down of the dot and the tap is not substantiallygenerated.

FIG. 19 shows the relationship between the dot and the tap according tothe third embodiment. The vertical axis shows the dot value and thehorizontal axis shows the tap value. Even in the embodiment, the tapvalue corresponding to one dot is 10. It is assumed that the tapadjusting circuit (delay line 117 in FIG. 5) can adjust the tap up to200 taps. That is to say, the light-emitting timing can be changed by200/10=20 dots to the downstream side in the light beam scanningdirection only by changing the tap value. In the invention, around theproximity of the center of the tap adjusting range is used in the mainscanning control to cope with the shift of the beam passing timing. Thatis to say, in the case that the adjusting range of the tap is 200 taps,the beam exposure position control is performed within ±100 tapsrelative to 100 taps of the center.

In FIG. 19, since the tap value corresponding to one tap is 10, theposition of n dots corresponds to (n−10) dots and 100 taps. The positionof the control value (0) corresponds to (n−10) dots and 109 taps, theposition of the control value (1) corresponds to (n−10) dots and 111taps, and position of the control value (2) corresponds to (n−10) dotsand 90 taps.

In the case that the exposure position of the light beam is changed bythe change in the temperature and the control value (1) becomes theobjective position of the control, only the tap is increased and thecontrol is performed by setting the dot and the tap to (n−10) dots and111 taps respectively. In the case that the control value (2) becomesthe objective position of the control, only the tap is decreased and thecontrol is performed by setting the dot and the tap to (n−10) dots and90 taps, respectively, as a matter of course.

Though the change in the temperature of the delay time of one tap issmall, when the carry-up/carry-down is performed, the shift of theexposure position is amplified to “the tap value corresponding to onedot×the amount of shift of one tap” at the maximum. However, accordingto the invention, since the carry-up/carry-down is not substantiallygenerated, the light beam scanning apparatus in which the shift in themain scanning direction is remarkably small can be provided.

FIG. 20 is the flow chart for determining the dot value n in the centerof the control range.

The CPU 51 sets the dot value n to zero (STEP 80). In this case, othernumbers in which the laser beam never reaches the timing sensor SR maybe used. Then, the CPU 51 sets a variable “a” to zero and sets a centralvalue T of the number of usable taps to 100 in this case (=200/10) (STEP81 and STEP 82). The CPU 51 turns on and scans the light beam to be thecontrol object in STEP 83, and the CPU 51 decides whether the countvalue of the timing sensor SR is not smaller than the predeterminedvalue M or not in STEP 84.

If the count value of the timing sensor SR is lower than thepredetermined value M (in the case of No in STEP 84), the CPU 51increases “a” by one, substitutes “a” for n (STEP 85 and STEP 86), andturns on and scans the light beam to be the control object (STEP 83).The dot value n in the center of the control range are determined andthe dot value is fixed to the determined value n in such a manner thatthe CPU 51 repeats STEP 83 to STEP 86 until the count value of thetiming sensor SR becomes not smaller than the predetermined value M.

FIG. 21 is the flow chart showing the main scanning control operationduring the standby state or the printing interval between papersaccording to the embodiment.

Since STEP 90 to STEP 94 are the same as STEP 20 to STEP 24 in FIG. 13,the description is omitted. If the count value of the output of thetiming sensor SR is not smaller than the predetermined value M (in thecase of YES in STEP 44), the CPU 51 decreases the tap value by one (STEP95). Accordingly, the CPU 51 shifts the exposure position by one tap tothe upstream side in the scanning direction. If the count value of theoutput of the timing sensor SR is smaller than the predetermined value M(in the case of No in STEP 44), the CPU 51 increases the tap value byone (STEP 96). Accordingly, the CPU 51 shifts the exposure position byone tap to the downstream side in the scanning direction.

As described above, the image forming apparatus according to theembodiment includes the exposure position setting portion (117, 119, and56) for setting the position of the exposure area EA exposed by the beamscanning of the scanning portion 35 as the additional value of the firstsetting value (dot value) which is set in the first unit (dot unit)corresponding to the scanning of the pixel clock period Pc and thesecond setting value (tap value) which is set in the second unit (tapunit) corresponding to the scanning of the period Δ·td smaller than thepixel clock period Pc.

The CPU 51 sets the second setting value (tap value) to the half of (apredetermined ratio to) the maximum value (200) of the second settingvalue (STEP 82), decides the first setting value (dot value) in whichthe position in the main scanning direction of the area scanned andexposed on the basis of the second setting value substantiallycorresponds to the objective position (STEP 85 and STEP 86), and fixesthe first setting value to the decided value. Further, the CPU 51changes the second setting value (tap value) of the exposure positionsetting portion according to the shift from the objective position ofthe exposure position EP during the standby state or the printinginterval between papers and corrects the shift (STEP 95 and STEP 96).

According to the embodiment, since the carry-up/carry-down of the dotand the tap is not substantially generated, the beam position error inthe main scanning direction can be decreased as small as possible.

1. An image forming apparatus comprising: a photosensitive member; abeam generating means for generating a light beam; a scanner whichreflects the light beam generated from the beam generating means towardthe photosensitive member and exposes the photosensitive member byscanning the photosensitive member in a main scanning direction with thelight beam; an exposure position setting means for setting a position ofan exposure area by beam scanning of the scanner as an additional valueof a first setting value which is set in a first unit corresponding tothe scanning of a pixel clock period and a second setting value which isset in a second unit corresponding to the scanning of the period smallerthan the pixel clock period; a first control means for controlling thebeam generating means and the scanner so as to expose the area set bythe exposure position setting means; a sensor which detects a positionin the main scanning direction of the area exposed by the first controlportion; a second control means for controlling the exposure positionsetting means so that the exposure position detected by the sensorbecomes an objective position, the second control means having a meansfor increasing the first setting value by one unit and changing thesecond setting value to a first predetermined value in the case that theadditional value of the exposure position setting means is increased toreach a first threshold, and a means for decreasing the first settingvalue by one unit and changing the second setting value to a secondpredetermined value in the case that the additional value of theexposure position setting means in which the first setting value hasbeen increased by one unit is decreased to reach a second thresholdsmaller than the first threshold, wherein the second threshold isseparated from the first threshold by a distance corresponding to avalue in which the first predetermined value is added to the value ofthe second unit corresponding to the scanning of the pixel clock period;a means for determining an image forming area on the photosensitivemember on the basis of the first setting value and the second settingvalue of the exposure position setting means and forming anelectrostatic latent image corresponding to input image data on theimage forming aria using the beam generating means; and a developerwhich develops the latent image formed on the photosensitive member withdeveloping agent.
 2. An image forming apparatus comprising: aphotosensitive member; a beam generating means for generating a lightbeam; a scanner which reflects the light beam generated from the beamgenerating means toward the photosensitive member and exposes thephotosensitive member by scanning the photosensitive member in a mainscanning direction with the light beam; an exposure position settingmeans for setting a position of an exposure area by beam scanning of thescanner as an additional value of a first setting value which is set ina first unit corresponding to the scanning of a pixel clock period and asecond setting value which is set in a second unit corresponding to thescanning of the period smaller than the pixel clock period; a firstcontrol means for controlling the beam generating means and the scannerso as to expose the area set by the exposure position setting means; asensor which detects a position in the main scanning direction of thearea exposed by the first control portion; a second control means forcontrolling the exposure position setting means so that the exposureposition detected by the sensor becomes an objective position, thesecond control means having a means for changing the second settingvalue to a value in which a multiple value of the second unitcorresponding to the scanning of the pixel clock period is multiplied bya multiple of a predetermined number of the first unit in the case thatthe second setting value is increased to a value more than the valuecorresponding to the multiple of the predetermined number of the firstunit; a means for determining an image forming area on thephotosensitive member on the basis of the first setting value and thesecond setting value of the exposure position setting means and formingan electrostatic latent image corresponding to input image data on theimage forming aria using the beam generating means; and a developerwhich develops the latent image formed on the photosensitive member withdeveloping agent.
 3. An image forming apparatus comprising: aphotosensitive member; a beam generating means for generating a lightbeam; a scanner which reflects the light beam generated from the beamgenerating means toward the photosensitive member and exposes thephotosensitive member by scanning the photosensitive member in a mainscanning direction with the light beam; an exposure position settingmeans for setting a position of an exposure area by beam scanning of thescanner as an additional value of a first setting value which is set ina first unit corresponding to the scanning of a pixel clock period and asecond setting value which is set in a second unit corresponding to thescanning of the period smaller than the pixel clock period; a firstcontrol means for controlling the beam generating means and the scannerso as to expose the area set by the exposure position setting means; asensor which detects a position in the main scanning direction of thearea exposed by the first control portion; a second control means forcontrolling the exposure position setting means so that the exposureposition detected by the sensor becomes an objective position, thesecond control means having a means for setting the second setting valueto a value of a predetermined ratio to the maximum value of the secondsetting value, deciding the first setting value in which the position inthe main scanning direction of the area exposed by the first controlportion substantially corresponds to the objective position, fixing thefirst setting value to the decided value, and changing the secondsetting value according to the shift of the exposure position from theobjective position; a means for determining an image forming area on thephotosensitive member on the basis of the first setting value and thesecond setting value of the exposure position setting means and formingan electrostatic latent image corresponding to input image data on theimage forming aria using the beam generating means; and a developerwhich develops the latent image formed on the photosensitive member withdeveloping agent.